Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of the limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or other organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Crystalline molecular sieves have a 3-dimensional, four-connected framework structure of corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known forms of molecular sieve are silicoaluminophosphate (SAPO) molecular sieves, which contain a three-dimensional microporous crystal framework structure of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units.
SAPO-34 and SAPO-18 are crystalline silicoaluminophosphate molecular sieve materials that have been reported as suitable catalysts for a variety of important processes, including the production of light olefins from oxygenates, such as methanol. SAPO-34 belongs to the family of molecular sieves having the framework type of the zeolitic mineral chabazite (CHA). SAPO-18 belongs to the family of molecular sieves having the AEI framework type. Other molecular sieves with the AEI framework type are ALPO-18 and RUW-18.
The preparation and characterization of SAPO-34 have been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 90(15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425-429 (1990).
The preparation and characterization of molecular sieves with AEI framework type have been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241-248 (1994); pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425-429 (1990); and U.S. Pat. No. 5,609,843.
In the conversion of methanol to olefins, SAPO-34 exhibits relatively high product selectivity to ethylene and propylene, and low product selectivity to paraffins and olefins with four or more carbons. Catalysts containing SAPO-34 are thus particularly suited for the conversion of methanol to olefins. Despite its good performance, carbonaceous deposits, commonly referred to as coke, quickly form in the catalytic cages of SAPO-34. Eventually, the presence of too much coke clogs up the cages and deactivates the catalyst. Also, despite its low selectivity to paraffins, SAPO-34 still produces by-products. Separating by-products from the desired ethylene and propylene adds additional cost to the methanol to olefin conversion process. Therefore, there is a continuing need to find new molecular sieves that have good product selectivity and produce few by-products.
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both CHA and AEI framework type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 20 values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown phases of AEI and CHA framework type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30, respectively (see FIG. 2 attached herewith).
U.S. Pat. No. 6,812,372, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, now indentified as EMM-2, comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein said intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve.
Prime olefin selectivity (POS), which equates to the total selectivity of ethylene and propylene in the product, and the prime olefin ratio (POR), which equates to the amount of ethylene divided by the amount of propylene in the product, are two of the main economic drivers in any oxygenate conversion process. In practice, even small changes in POS and/or POR have an enormous effect on the economics of a commercial process. According to the present invention, it has now been found that, for certain AEI/CHA intergrowth materials, there is an optimum range of silica/alumina ratio over which both the POS and POR are maximized when the materials are used in the conversion of oxygenates to olefins. In addition, these AEI/CHA intergrowth materials have been found to exhibit excellent hydrothermal stability.